1. Field of the Art
This invention relates generally to ethylene polymer compositions. More particularly, it relates to an ethylene polymer composition comprising a copolymer comprising, as predominant monomeric units, ethylene and an olefinically unsaturated silane compound, a silanol condensation catalyst, and a flame retardant. The flame-retardant ethylene polymer composition of the present invention is suitably used, for example, for covering electric wires.
Conventional flame-retarded polyethylene resin compositions prepared by adding, to polyethylene resins, organic halogen compounds such as chlorinated paraffin or chlorinated naphthalene and antimony oxide and the like have been used for wiring materials within various apparatus and equipment, electric insulation materials, domestic articles, and other parts where there is a risk of over-heating or fire.
However, these conventional flame-retardant polyethylene resin compositions, the polyethylene resin component of which is not crosslinked, are readily heat-deformed or melted at the softening point of polyethylene or a higher temperature. The mechanical properties of such compositions deteriorate rapidly at such temperatures.
When these conventional compositions are used for wire-covering and the like, the above-mentioned problem is apt to give rise to other undesired problems such as thermal deformation and melting of the covering materials, due to the heat for soldering wiring systems within various equipment or to over-current heat due to equipment trouble. Thus, the imparting of heat resistance to these materials has been an important object to be achieved.
As one of the measures directed toward solving these problems, the use of a crosslinked polyethylene is generally practiced.
2. Prior Art
In one of such crosslinking methods, the polyethylene to be used is modified into a crosslinkable polyethylene by introducing a crosslinking group thereinto. In this case, it is also known to utilize a hydrolyzable silane group as the crosslinking group. This method comprises reacting polyethylene with an unsaturated silane compound in the presence of a free-radical yielding agent to obtain a graft copolymer and then contacting the resulting graft copolymer with moisture in the presence of a silanol condensation catalyst to effect crosslinking.
This method involves certain problems in that an additional step is required to graft-polymerize the unsaturated silane compound in advance, and a general-purpose polyethylene cannot be used as it is. The method, however, is advantageous on the following points over other crosslinking means such as irradiation or the use of free radicals formed by decomposition of organic peroxides and the like.
For example, the irradiation method requires a costly apparatus of special type and, moreover, it is actually not practicable for producing thick-walled or complicated molded articles. The method using organic peroxides requires a precise control between the activation temperature of organic peroxides adn the softening temperature of polyethylene resins. Since such a critical control of processing conditions may often be unachievable, the resulting resin compositions are apt to result in molded products of poor quality and sometimes result in failure of molding operations.
When the polyethylene graft-polymerized with the unsaturated silane compound is used, there has also been a problem in that the moldability of the resulting resin compositions is not always satisfactory and expecially unsatisfactory when flame retardants are incorporated into the compositions. The reasons for these problems have not been fully elucidated but are considered to be due to occurrence of unusual crosslinking reactions in the course of molding. Also in this case, there has been a problem in that the resulting resin compositions often result in the unsatisfactory appearance of molded products and sometimes result in the failure of molding operations.